An antenna sharing device for use with a dual-mode mobile phone compatible with GSM and W-CDMA systems has been known in which: a diplexer separates a frequency band for GSM and a frequency band for W-CDMA; transmit and receive signals in the GSM band are separated using a high-frequency switch for GSM; and transmit and receive signals in the W-CDMA band are separated using a duplexer for W-CDMA (see, for example, U.S. patent application Publication No. 2004/0240420 A1).
Another antenna sharing device has been known in which, in order to use frequency bands for GSM, DCS, PCS, and W-CDMA: a diplexer separates a low-frequency band in which GSM transmit and receive signals are included and a high-frequency band in which DCS, PCS and W-CDMA transmit and receive signals are included; the GSM transmit and receive signals are separated using a first high-frequency switch; and the DCS and PCS transmit signals, the DCS and PCS receive signals, and the W-CDMA transmit signal are separated using a second high-frequency switch (see, for example, Japanese Patent Laid-Open No. 2004-40322). There has also been a wireless telephone terminal having a high-frequency signal processing circuit in which, in order to use frequency bands for EGSM, GSM850, GSM, DCS, and PCS: switching elements included in a switching circuit on the high-frequency side for handling high-frequency signals separated, by a diplexer, from low-frequency signals are made of high-frequency compound semiconductor transistors and switching elements included in a switching circuit on the low-frequency side for handling low-frequency signals separated, by the diplexer, from the high-frequency signals are made of PIN diodes (see, for example, Japanese Patent Laid-Open No. 2004-193866). There has also been a high-frequency device in which a varistor and an inductor are connected in parallel with a signal line between an antenna terminal and a diplexer (see, for example, U.S. Patent Application Publication No. 2004/0087286 A1).